A thought occurred to me when I noticed that the Druid can learn the Polymorph spell and I came up with the idea of casting Polymorph on yourself to turn into beast with a challenge rating that matches your level instead of only turning into a beast with a challenge rating of your level divided by 3.

1 Answer
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Yes, a druid can cast polymorph on themselves.

In case you are wondering whether the caster can only target creatures other than themselves, the Players Handbook addresses that:

Targeting Yourself
If a spell targets a creature of your choice, you can choose yourself, unless the creature must be hostile or specifically a creature other than you. If you are in the area of effect of a spell you cast, you can target yourself.

(Basic Rules, p80, PHB, p204.)

There are some limitations. The new form has to have the type of beast, and, as you noted, the CR needs to be less than or equal to your level. And when you transform you assume all of the game characteristics of the new form except for personality and alignment.

It's probably worth a conversation with your GM about what beasts you can transform in to, and how much you need to know about them. It's probably no big deal to transform into a beast you know, like a cat (assuming your character knows about cats), but your GM might have issues if you want to transform into a sabertooth tiger, for instance, if your character does not know what that creature is.

One concern might be whether you can maintain concentration in the new form. This answer answers that fairly authoritatively.

It's worth noting that polymorph is different in almost every particular from the druidic class feature Wild Shape. Differences include concentration not needed for Wild Shape, CR of target beast, length of time changed, and details of game statistics replaced, among others.